Prepare grill to temperature.
If the sausage is in bratwurst form, slice casings lengthwise and empty the ground sausage meat into a large mixing bowl. Form ground sausage into 8 patties and set aside.
Slice yellow onion thinly into rings. Add flour and seasonings. Mix well into the onions to coat each ring evenly.
In a medium skillet, warm peanut oil on the stove to 350°F. Add onions and fry until brown in color, then remove from oil and place freshly fried onions on paper towels to drain and cool.
Place brat sausage patties on the grill and heat about 6 to 8 minutes per side, checking for doneness. When done, the patties should have no pink remaining in the center and reach an internal temperature of 165°F.
As patties are nearing completion, place two slices of Cooper Sharp cheese on top of each grilled patty, crisscrossed. Heat until cheese is melted (cover to speed up melting). Remove from grill and set aside.
Place pretzel rolls on grill and toast lightly.
Assemble your burgers:
Spread the bottom half of each pretzel roll with Dijon mustard. Layer some shredded cabbage onto bottom bun. Add a brat cheeseburger patty. Place a nice portion of fried onions on the burger and cap with the remaining half of the pretzel roll.
Serve any remaining batter-fried onions on the side, using extra Dijon mustard for dunking.